The present invention relates to an oscillation circuit, an electronic apparatus, and a timepiece.
The oscillation circuit used in a portable wristwatch or electronic apparatus often uses a battery or a rechargeable secondary battery as a main power source to drive electronic circuitry. The electronic circuitry that is used in such an appliance often creates a reference clock from the oscillation frequency fs of the oscillation circuit.
An example of a conventional oscillation circuit is shown in FIG. 1.
In this figure, a main circuit portion 20 of an oscillation circuit is formed on a semiconductor substrate, and this main circuit portion 20 is connected to the two ends of a crystal oscillator 10 by input-output terminals Xin and Xout that are provided for a signal circuit.
The main circuit portion 20 comprises an inverter 22 connected by a signal path to the crystal oscillator 10 and a feedback resistor 24 connected to the input and output sides of the inverter 22.
Since the input terminal Xin of the crystal oscillator 10 is connected directly to the input side of the inverter 22 in this conventional oscillation circuit, if there is any change in the potential of the input terminal Xin of the crystal oscillator 10, a waveform in which the potential has changed is input directly to the inverter 22. If this input waveform does not cross the threshold voltage of the inverter 22 during this time, the operation of the oscillation circuit will halt.
If a fault such as a leak should occur between the input terminal Xin of the crystal oscillator 10 and the power source in this conventional oscillation circuit, changing the potential on the input side of the inverter 22, therefore, problems could occur such as a halting of the oscillation or large variations in the oscillation frequency if the oscillation does not actually stop.
In particular, since this main circuit portion 20 of the oscillation circuit is formed on the semiconductor substrate and the crystal oscillator 10 is attached externally, leakage can easily occur at the input terminal Xin that is the connection therebetween, making countermeasures necessary.
An example of a conventional oscillation circuit that uses a DC-cutting capacitor 26 as means for preventing the stopping of oscillation due to leakage is shown in FIG. 2.
In this conventional example, the DC-cutting capacitor 26 is connected between the input terminal X and the input side of the inverter 22 in the signal path.
The input terminal Xin of the crystal oscillator 10 and the input side of the inverter 22 are galvanically separated by this DC-cutting capacitor 26. In addition, the waveform that is input to the inverter 22 is a waveform that has been charged and discharged by the DC-cutting capacitor 26. For that reason, since the waveform that has been charged and discharged in the DC-cutting capacitor 26 crosses the threshold voltage of the inverter 22, the oscillation of the oscillation circuit does not halt, even if the potential of the input terminal Xin should change due to a leak or the like. In other words, it is possible to implement an oscillation circuit that operates stably with no problems such as oscillation halt, even if a leak should occur between the input terminal Xin of the crystal oscillator 10 and the power source.
However, if the DC-cutting capacitor 26 is provided on the input terminal Xin side of the signal path as shown in FIG. 2, the potential of the input terminal Xin of the crystal oscillator 10 will be close to the open state, which is extremely unstable. Moreover, any change in the potential of the input terminal Xin of the crystal oscillator 10 will cause a change in each depletion layer of the parasitic capacitances Cy1, Cy2, and Cx of the main circuit portion 20, changing the capacitances thereof.
Therefore, if a slight leak occurs at the input terminal Xin of the crystal oscillator 10 due to an external disturbance such as an increase of humidity or light, changing the potential of the input terminal Xin, the parasitic capacitances will change accordingly. As a result, the oscillation constant of the oscillation circuit will change, the oscillation frequency itself will change, and a problem will occur in that the operation of the electronic circuitry that uses that oscillation frequency as a reference clock will be adversely affected.
In particular, if the DC-cutting capacitor 26 of the conventional oscillation circuit is provided on the semiconductor substrate, a circuit configuration is created in which the parasitic capacitance Cx that is generated thereby is positioned on the input terminal Xin side, so that the previously-described generation of the minute leakage current causes variations in the magnitude of the parasitic capacitance Cx, which leads to large variations in the parasitic capacitance of the entire circuit, which causes a problem in that it results in large variations in the oscillation frequency.